Carbon fiber prepreg and carbon fiber reinforced resin composite

ABSTRACT

A carbon fiber prepreg comprising a unit layer consisting of carbon fibers and a matrix resin layer, and stranded filaments or single filaments incorporated thereinto. The steel fiber is an Ni plated low-carbon dual-phase steel filament and has a filament diameter of not more than 100 μm, a tensile strength of 300 to 700 kgf/mm 2 , and an area reduction at fracture of more than 20%. The carbon fibers and steel filaments are aligned codirectionally, and the steel fiber content of the prepreg relative to the carbon fiber is not more than 10 vol %. Sheets of the carbon fiber prepreg are laminated to form a carbon fiber reinforced composite.

TECHNICAL FIELD

The present invention relates to a prepreg comprising a matrix resinlayer and carbon fibers oriented unidirectionally therein, and acarbon-fiber reinforced resin material made of laminated layers of sucha prepreg. More particularly, the invention relates to a prepreg and/orresin material of the type which has excellent strength and elasticitycharacteristics and yet provides improved impact resistance andtoughness, and which provides improved wettability and adhesivityrelative to the matrix resin, thus enhancing its quality reliability.

BACKGROUND ART

Carbon-fiber reinforced plastics (hereinafter referred to as CFRP) havegood advantages over other fiber reinforced plastics in mechanicalcharacteristics, such as strength, modulus of elasticity, and lightness.For this reason, CFRP has found use in various applications in suchfields as aircraft, spacecraft, sports, and equipment for leisure timeamusement, and indeed the demand therefor is on the increase.Conventionally, however, CFRP has a drawback that it is not wellqualified in respect of impact resistance and toughness, though it hashigh strength and high modulus of elasticity. As such, improvement inthis respect is required in order that CFRP may be found satisfactoryfor use as a structural material for aircrafts and the like.

In order to overcome such a drawback of CFRP in respect of impactresistance and toughness, there have been developed hybrid plasticscomprising carbon and aramid fibers. Also, as described in JapanesePatent Laid-Open Publication No. SHO 58-90943, there has been proposed afiber reinforced composite material comprising carbon fiber and aromaticpolyamide fiber.

However, with the known hybrid plastics, one problem is that in order toobtain improved toughness and impact resistance, the hybrid plastic mustcontain a considerably large amount of aramid fiber, which may adverselyaffect the elastic property feature which is characteristic of carbonfiber. Another problem is that the aramid fiber component exhibits poorperformance in respect of wettability and adhesivity relative to thematrix resin, which fact may easily affect the rigidity and strengthcharacteristics of the hybrid plastic, thus lowering the qualityreliability of the plastic.

In the above cited Japanese Patent Laid-Open Publication No. SHO58-90943, it is stated that the fiber reinforced composite materialcomprising carbon and aromatic polyamide fibers utilizes respectiveadvantages of the two kinds of fibers, whereby respective shortcomingsof the two fibers, i.e., insufficient impact resistance of the carbonfiber and low elasticity modulus of the aramid fiber, can besatisfactorily overcome, and more particularly improvement can beobtained in impact resistance and in adhesion property relative to thematrix resin, over "KEVLAR" (Registered trademark of Du Pont), a typicalaramid fiber. However, this fiber reinforced composite material stillhas a problem in that its mechanical characteristics, such as strength,elasticity modulus, and impact resistance, are insufficient for use inthe above mentioned applications, and improvement in this respect hasbeen demanded.

It is a primary object of the invention to provide a carbon fiberprepreg and a carbon fiber reinforced material made from the prepregwhich provide improved impact resistance and toughness and improvedwettability and adhesivity relative to the matrix resin, withoutdetriment to the outstanding characteristics of the carbon fibercomponent, and which can thus exhibit improved quality reliability andmeet the foregoing demand in respect of all the performancecharacteristics required of fiber reinforced materials in general.

DISCLOSURE OF THE INVENTION

According to one aspect of the invention, as claimed in claim 1, thereis provided a carbon fiber prepreg comprising a unit layer consisting ofa matrix resin layer and carbon fibers unidirectionally buried therein,and stranded filaments formed from steel fibers having a filamentdiameter of not more than 100 μm, a tensile strength of 300 to 700kgf/mm², and an area reduction at fracture of more than 20% which aremixed with the carbon fibers codirectionally with the carbon fibers sothat the steel fiber content of the prepreg relative to the carbonfibers is not more than 10 vol %.

In its second aspect, as claimed in claim 2, the invention provides acarbon fiber prepreg wherein single filaments of the steel fiber definedin claim 1 are mixed with the carbon fibers codirectionally therewith insuch a way that they are evenly and finely dispersed.

The invention, in another aspect thereof as claimed in claim 3, providesa carbon fiber reinforced resin composite comprising layers of thecarbon fiber prepreg as defined in claim 1 or 2 which are laminatedtogether. In a further aspect of the invention, as claimed in claim 4,the steel fibers are plated. In another aspect of the invention asclaimed in claim 5, the steel fibers are of a low-carbon dual-phasesteel. In another aspect of the invention, as claimed in claim 6, theplating applied to the steel fibers is Ni plating.

The prepregs of the invention include one having carbon and steel fibersseparately arranged within the matrix resin layer, and one havingpre-doubled filaments of the two kinds of fibers arranged within thematrix resin layer.

The steel filaments may be arranged in mixture with the carbon fibers,either within the unit layer comprising the carbon fibers and the matrixresin layer or between such unit layers. It is to be noted, however,that where the steel filaments are arranged between unit layers, unevenstress is likely to develop between the unit layers, with the resultthat the effect of the prepreg may be reduced. Therefore, it ispreferred that the steel filaments should be arranged within the unitlayer. The steel filaments may be arranged either equidistantly orrandomly.

Stranded filaments that may be used in the present invention include notonly those formed by twisting a plurality of steel filaments twistedtogether, but also so-called doubled steel fiber yarns. For the steelfibers, not only low-carbon dual-phase steel filaments, but also pianowires and stainless steel filaments may be used. The term "low-carbondual-phase steel filament" referred to herein means a steel filament,earlier proposed by the present applicant, which is produced bysubjecting a wire rod of 0.01 to 0.50 mm in diameter which contains, interms of wt %, 0.01 to 0.50% of C, not more than 3.0% of Si, and notmore than 5.0% of Mn, with the rest consisting of Fe and unavoidableimpurities, to a primary heat treatment, a primary cold drawing, asecondary heat treatment, and a secondary cold drawing so that the wirerod is forcedly worked for wire diameter reduction to not more than 100μm (see Japanese Patent Laid-Open Publication No. SHO 62-20824). Alow-carbon dual-phase steel filament produced according to the abovedescribed method has a fibrous fine metal texture in which cells workedby such forced working are unidirectionally fibrously oriented such thatthe size of worked cells is of the order of 5 to 100 Å, with aninterfiber spacing of 50 to 1000 Å, the steel filament having a tensilestrength of more than 300 kgf/mm². Where such a low-carbon dual-phasesteel filament as a metal fiber component, addition of the steelfilament in a very small amount provides increased fiber elongation atbreak and can substantially improve the impact resistance of theprepreg.

For the matrix resin of the invention, thermosetting resins, such asepoxy resin and phenolic resin, and thermoplastic resins, such aspolyester and polyamide, may be used.

The reinforcing mechanism of the invention will now be explained. Thestrength of a composite material is generally expressed as follows:

Composite material strength=reinforced material strength×vol % +matrixstrength×vol % That is, the strength of the composite material dependson the strength and volume percentage of the reinforcing material, andaccordingly the higher the strength of the reinforcing material, thesmaller is the volume percentage of the reinforcing material.

Where reinforcing materials, such as conventional KEVLAR and aramidfibers, are used, in order to increase the strength of the compositematerial by a few percent, it is necessary that these fibers must beadded on the order of a few percent correspondingly. In other words,unless a corresponding vol % of fibers is added, no strength increase isobtainable. This is a negative factor from the standpoint of weightreduction. In contrast, the steel fiber used in the invention has theadvantage that only 1% addition of same results in 20% increase instrength and addition of only about 0.5% results in a strength increaseof the order of 10%. Such high increase in strength is unconceivablefrom what is commonly known with conventional composite materials.Furthermore, use of the steel fiber can result in a very noticeableimprovement in fracture toughness (Charpy impact strength), theinsufficiency of which characteristic has been a fatal drawback ofconventional carbon fiber prepregs and/or reinforced resin materials.

This is a unique advantage of the present invention which cannot beexplained merely by the foregoing principle of strength improvement.When a large external force is applied on any conventional CFRP,fracture is caused to carbon fiber components which are inherentlyfragile and the fracture, mainly in the form of a resin crack,propagates in a direction perpendicular to the carbon fiber, whicheventually leads to composite material fracture. In contrast, in theprepreg of the present invention the presence of steel fibers havingaforementioned characteristics helps stop the propagation of cracking;or the toughness and strength of the steel fibers per se serve toprevent the fracture of carbon fibers per se. It is believed that suchresisting behavior of the steel fibers contributes toward overallstrength improvement. Therefore, the term "steel fiber" referred toherein means a steel fiber which must have good toughness in itself andwhich has an area reduction at fracture of more than 20%, preferablymore than 50%. For example, a low-carbon dual-phase steel filament, apiano wire, or a stainless steel filament, as indicated in Table 2, maybe used as such. High strength alone of the steel fiber may contributetoward weight reduction, but unless it also has good toughness, thesteel fiber does not serve to prevent the fracture of the carbon fibercomponent, it being thus impracticable to obtain such outstandingfracture toughness of the prepreg of the invention. KEVLAR and aramidfibers, for example, have a toughness of the order of 5% in terms ofat-fracture reduction of area, which cannot give any such good effect.The same is true with other metal filaments, such as tungsten andtitanium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view for explanation of a carbon fiber prepregrepresenting a first embodiment according to one aspect of the inventionas claimed in claim 1;

FIG. 2 is a perspective view showing a carbon-fiber reinforced resincomposite made of the carbon fiber prepreg of the first embodiment;

FIG. 3 is a sectional view showing steel fibers and a stranded filamentin the first embodiment;

FIG. 4 is a schematic view showing an apparatus for production of thecarbon fiber prepreg of the first embodiment;

FIG. 5 is a perspective view showing a modified form of the firstembodiment;

FIG. 6 is a perspective view showing another modified form of the firstembodiment;

FIG. 7 is a perspective view for explaining layers of a carbon fiberprepreg, and also of a carbon-fiber reinforced resin composite,representing a second embodiment according to another aspect of theinvention as claimed in claim 2;

FIG. 8 is a perspective view showing a modified form of the secondembodiment;

FIG. 9 is an exploded view in perspective showing test pieces used in atest conducted in ascertaining the performance effect of the secondembodiment; and

FIG. 10 is a characteristic view showing the results of the testsconducted with respect to the second embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will now be described with reference to theaccompanying drawings.

First Embodiment

FIGS. 1 through 3 are views for explaining a carbon fiber prepreg and acarbon-fiber reinforced resin material made therefrom, both representingthe first embodiment according to one aspect of the invention as claimedin claim 1.

In the drawings, the reference numeral 1 designates a carbon-fiberreinforced resin composite which is a laminate formed by laminating aplurality of layers of a carbon fiber prepreg 2 and heating andhardening the same.

The carbon fiber prepreg 2 comprises stranded filaments 4b formed bytwisting a plurality of carbon fiber stranded filaments 4a and ten-oddsteel fiber filaments 5 together, and a matrix resin layer 3 in whichare buried the stranded filaments at predetermined intervals. The carbonfiber stranded filaments 4a and steel fiber stranded filaments 4b arecodirectionally oriented within the prepreg 2, and the carbon-fiberreinforced resin composite 1 consists of layers of such carbon fiberprepreg 2 which are laminated in such a way that the carbon fiberstranded filaments 4a and steel fiber stranded filaments 4b inrespective layers are codirectionally oriented, the steel fiber 5content of the resin composite 1 being 0.5 to 10 vol % relative to thecarbon fiber content.

The steel fiber 5 is a low-carbon dual-phase steel fiber filament of 10to 40 μm in diameter having the above described composition and producedaccording to the above described method. Therefore, the low-carbondual-phase steel fiber filament has a fibrous fine texture in whichcells worked through the above described forced working areunidirectionally fibrously arranged. The size of the worked cells andthe spacing between adjacent filaments are of the order of 5 to 100 Åand 50 to 1000 Å respectively. Further, the steel fiber filament has atensile strength of 300 to 700 kgf/mm², an elasticity modulus of 15 to25 ton/mm², and an elongation of 2 to 5%. An Ni plated layer 6 is formedon the outer surface of the steel fiber 5. The Ni plated layer is formedby plating the above mentioned wire rod and has a deformation that hasbeen created by plastic working carried out simultaneously as the wirerod was subjected to cold wire drawing.

The method employed in fabricating the carbon fiber prepreg 2 and carbonfiber reinforced resin material 1 will now be described.

FIGS. 4 (a) and 4 (b) are schematic views showing an apparatus forfabricating a carbon fiber prepreg 2. The apparatus 10 comprises threesets of creels 12 for separately feeding strands of a carbon fiber yarn(carbon fiber stranded filament) 11 formed from a multiplicity of carbonfibers stranded together and strands of a steel fiber yarn (steel fiberstranded filament) 24 formed from a plurality of steel fibers strandedtogether, a slit station 12 for uniformly orienting strands of carbonfiber yarn 11 supplied from respective creels 12 and for arrangingstrands of steel fiber yarn 24 at predetermined intervals, a pair ofrolls 16, 16 for thermocompression bonding of release paper 14 andrelease film 15, both coated with epoxy resin, onto carbon fiber yarn 11and steel fiber yarn 24, and a drum 17 for taking up a carbon fiberprepreg 18 thus formed.

In producing a carbon fiber prepreg 2 by using the apparatus 10, carbonfiber yarn 11 and steel fiber yarn 24 are separately fed from respectivecreels 12, and strands of the carbon fiber and steel fiber yarns 11 and12 are evenly aligned. Then, release paper 14 is thermocompressionbonded to the underside of both yarns 11 12 and release film 15 islikewise bonded to the upper surface thereof, whereby a prepreg sheet 18is formed, the prepreg sheet 18 being then wound onto the drum 17.

The prepreg sheet is then cut to a predetermined length for forming acarbon fiber prepreg 2. A predetermined number of layers of the prepregis laminated to form a laminate. Subsequently, the laminate is heatedand hardened, whereby a carbon fiber reinforced resin composite 1 isproduced.

The method for such fabrication is not limited to the above describedone in which carbon fiber yarn 11 and steel fiber yarn 24 are separatelysupplied. A single form of yarn strand comprising the two fiber yarns11, 24 stranded in a doubled fashion may be supplied and arranged onrelease paper 14, or such form of yarn strand may be continuously woundonto a drum in a wet process using organic solvent.

In this way, the present embodiment is of such arrangement that carbonfiber stranded filaments 4a and steel fiber stranded filaments 4b areburied in a matrix resin layer 3 in which they are codirectionallyoriented. According to this arrangement, it is possible to improve theimpact resistance and toughness of the prepreg 2 by virtue of the steelfiber 5 while preserving the high strength and high elasticitycharacteristics of the carbon fiber, and further to strikingly improvethe strength of the carbon fiber stranded filaments 4a and steel fiberstranded filaments 4b against axial tension, bending force and so on. Aplurality of such carbon fiber prepregs 2 are superposed to form acarbon fiber reinforced resin composite 1. The resin composite cantherefore be successfully employed for use as a structural material foraircrafts and the like, which area of application has hitherto beenfound difficult for entry, it being thus possible to meet theaforementioned demand.

In the present embodiment, for the steel fiber 5 is employed alow-carbon dual-phase steel filament. This provides for furtherimprovement in all the required mechanical characteristics, such astensile strength, elasticity, and elongation. Furthermore, an Ni platedcover layer 6 is formed on the low-carbon dual-phase steel filament,which can result in improvement in the adhesivity and wettability of thesteel filament relative to the matrix resin layer 3 and thus in improvedquality dependability.

Furthermore, the present embodiment can minimize the quantity of steelfiber stranded filament 4b contained in the carbon fiber prepreg 2 and,in turn, reduce such filament content of the resin material 1. Thus, itis possible to enhance hybridization without detriment to the inherentcharacteristics of carbon fiber, such as high modulus of elasticity, andto avoid any weight increase inherent in the use of metal fibers, thuscoping with the demand for weight reduction.

Experiment 1

An experiment was carried out to ascertain the characteristic enhancingeffect of the carbon fiber reinforced resin composite of the presentembodiment, which experiment will be explained below. In the experiment,two types of embodiment samples 1, 2 were prepared according to thefabrication procedures employed for the present embodiment. Also, threetypes of reference samples 1-3. Measurement was made with these samplesin respect of tensile strength, tensile modulus of elasticity,elongation at rupture, bending strength, and Charpy impact strength, andmeasurements obtained with respect to respective samples were compared.

A carbon fiber prepreg was prepared according to the above describedmethod of fabrication and by using a carbon fiber yarn formed of 6000filaments and a steel fiber yarn formed of 8 filaments. For the epoxyresin was used a mixture of epichlorohydrin-bisphenol A type liquidresin, solid resin of same type, and phenolic novolak type liquid resin,and for the hardening agent were used dicyandiamide (DICY) and3-(P-chlorophenyl)-1,1-dimethyl urea (DCMU) in combination. Apredetermined number of aforesaid carbon fiber prepregs was laminated,and the laminate was subjected to heating and hardening in a compressionmolding machine under the conditions of 130° C.×2 hr and 8 kg/cm²,whereby a carbon fiber reinforced plastic was produced (EmbodimentSample 1).

Another carbon fiber reinforced plastic was produced according to thesame method of fabrication and under the same conditions, except thatthe number of filaments in the steel fiber yarn was 18 (EmbodimentSample 2).

Nextly, for purposes of comparison, a carbon fiber yarn of 6000filaments and a steel fiber yarn of 8 filaments were woven into a fabric(of plain weave) so that the unit number of warp and weft yarns was 9/25mm. The fabric was treated in a wet process wherein it was steeped inthe above mentioned resin which was diluted with solvent. Thereafter,the fabric was heated and formed into a carbon fiber prepreg. Pluralsheets of the prepreg were laminated, and the laminate was subjected tocompression molding under the same conditions as above, whereby a carbonfiber reinforced plastic was produced (Reference Sample 1). By usingaramid fiber yarn (of 50 filaments; of same volume as metal fiber yarn)in place of the steel fiber yarn, an aramid fiber hybrid carbon fiberreinforced plastic was produced in same manner as the method employed inmaking Embodiment Sample 1 (Reference Sample 2). In addition, a carbonfiber reinforced plastic comprising carbon fiber alone was prepared(Reference Sample 3).

Table 1 shows the results of measurements made with respect to thesesamples. The first column of the table gives steel fiber of aramid fibercontent relative to the carbon fiber.

As is apparent from the table, Reference Sample 1 which incorporatessteel fiber as a yarn component of a woven fabric exhibits goodworkability, but all its mechanical characteristics are found very low.Reference Samples 2, 3 are found satisfactory in elastic modulus intension, at 1.15 and 1.22 GPa respectively; in bending strength, at 1420and 1513 MPa respectively; and in bending modulus of elasticity, at 1.06and 1.18 GPa. However, measurements with respect to these samples werefound unsatisfactory in tensile strength at 1230 and 1201 MParespectively; in elongation at rupture at 0.92% and 0.89% respectively;and in Charpy impact strength at 0.93 and 0.90 kgfm/cm². In contrast,measurements with respect to Embodiment Samples 1 and 2 indicate thatthe samples are satisfactory in tensile modulus in tension, at 1.23 and1.23 GPa respectively; in bending strength, at 1514 and 1543 MParespectively; and in bending modulus of elasticity, at 1.81 and 1.10 GParespectively. Further, remarkable improvement is witnessed in tensilestrength, at 1469 and 1540 MPa; in elongation at rupture, at 1.09% and1.10% respectively; and in Charpy impact strength, at 1.13 and 1.21kgfm/cm². It an be seen from these results that unidirectionalhybridization of steel fiber provides for well balanced improvement invarious mechanical characteristics without detriment to the inherentcharacteristics of carbon fiber and through a slight addition of steelfiber.

Table 3 gives Charpy impact test values, and mean values of same,measured at Charpy impact tests conducted with respect to referencesample pieces which comprise carbon fiber alone, and embodiment samplepieces containing low-carbon dual-phase steel fiber stranded filaments(of about 1 vol %). As is clearly seen from the table, whereas in thecase of reference sample pieces containing carbon fiber alone, meanvalue is registered at 0.95 kgfm/cm², the mean value with respect to theembodiment sample pieces is registered at 1.02 kgfm/cm², or animprovement on the order of 7%.

Various forms of steel fiber arrangement may be used. For example, asshown in FIG. 5, outer surface portions only may be reinforced withsteel fiber stranded filaments 4b, with carbon fiber stranded filaments4a arranged only in interior portions. This can be achieved bylaminating together prepregs 2 having stranded filaments 4b arrangedthroughout the entirety thereof and prepregs 20 having such filamentsarranged at opposite side portions thereof. For the directions in whichcarbon fiber prepregs are laminated, various forms of lamination may beemployed. For example, as FIG. 6 shows, lamination may be carried out insuch a way that directions of fiber orientation are orthogonal to eachother or intersect at an angle.

Embodiment 2

FIG. 7 is a view for explaining a carbon fiber prepreg representing asecond embodiment of the invention, as claimed in claim 2, and a carbonfiber reinforced resin composite using same.

In the figure, the reference numeral 30 designates a carbon fiberreinforced resin composite which is fabricated by laminating pluralsheets of carbon fiber prepreg 31 together and then by heating andhardening the laminate. The carbon fiber prepreg 31 comprises a matrixresin layer 34 and carbon fiber filaments 32 and steel fiber filaments33 uniformly and finely dispersed within the matrix resin layer in whichthey are buried. The carbon fiber filaments 32 and steel fiber filaments33 are codirectionally arranged, and the steel fiber 33 content of thecarbon fiber reinforced resin composite 30 is 0.5 to 10 vol % relativeto the carbon fiber 32. Further, for the steel fiber component 33 isused a low-carbon dual-phase steel fiber filament having an Ni platedsurface.

In the present embodiment, steel fiber filaments 33 are buried withinthe matrix resin layer 34, with the filaments uniformly and finelydispersed therein. According to such arrangement, it is possible toefficiently improve the impact resistance and toughness of the carbonfiber reinforced resin material while preserving the high strength andhigh elasticity characteristics, and also to provide for characteristicequalization. For reference, it is noted that where stranded filamentsare buried in spaced apart relation as in the embodiment representing afirst aspect of the invention, as claimed in claim 1, the average valueof such characteristics as impact resistance and toughness can beimproved when the characteristics are viewed as a whole, but locallythere are portions in which steel fiber is not present and nocharacteristic improvement has occurred with respect to such localportions, which fact presents a limitation from the standpoint ofcharacteristic equalization. In view of this fact, the presentembodiment is of such arrangement that steel fiber filaments are evenlyand finely distributed so that characteristic equalization may be easilyachieved. Depending upon the application and the effect of the loadapplied, there may be cases where only a certain local portion should becharacteristically improved. In the present embodiment, steel fiberfilaments are arranged in a portion or portions which requirecharacteristic improvement. It is thus possible to provide efficientimprovement in the required characteristics and eventually to contributeto weight reduction.

Experiment 2

Impact tests were carried out to ascertain the effect of the carbonfiber reinforced resin composite of this second embodiment, which testswill be explained below. In these tests, as FIG. 9 shows, first tosixth, eighth, tenth, and eleventh unit layers containing carbon fibersonly, and seventh and ninth unit layers having low-carbon dual-phasesteel filaments uniformly and finely distributed therein, weresequentially laminated to form test specimens of 10 mm width×1.6 mmthickness×80 mm length each which represent the present embodiment.Impact strength was measured with respect to each specimen, where thespecimen had a low-carbon dual-phase steel filament content of 1.4 vol%, the filament diameter being 50 μm one case and 100 μm in another case(specimen Nos. 2-1, 2-2); and where the specimen had such a content of2.8 vol %, the filament diameter being 50 μm in one case and 100 μm inanother case specimen Nos. 3-3, 3-2) as shown in Table 4. For purposesof comparison, measurement was made likewise with respect to aconventional test specimen consisting of first to eleventh unit layers(specimen No. 1). Impact tests were carried out employing a Charpyimpact tester (5 kgf.m) and according to the flatwise impact testprocedure, with a hammer lifting angle set at 90 degrees (2.5 m/S) andan interspan distance set at 60 mm. Impact was applied from the eleventhlayer side.

As is apparent from Table 4 and FIG. 10, with conventional specimen (No.1), impact values are registered at 1.74 to 1.98 kgfm/cm², averaging at1.84 kgfm/cm². In contrast, with embodiment specimens ("SCIFER";Registered trademark in Japan;Nos. 2-1, 2-2, 3-1, 3-2), impact valuesaveraged at 2.09 to 2.29 kgfm/cm², or an improvement of 14 to 24% overthe conventional specimen. According to the results of the tests, thegreater the low-carbon dual-phase steel filament content, the higher isthe impact value, and further improvement is seen as compared with thecase where stranded filaments are buried in the matrix resin layer.

The form of steel fiber arrangement may be varied in different ways. Forexample, as shown in FIG. 8, only surface portions on which impact loadis likely to be applied may be reinforced with steel fiber 33, with onlycarbon fiber arranged in the interior portion. Where such arrangement isused, further weight reduction can be achieved while improvement isobtained in impact resistance.

INDUSTRIAL APPLICABILITY

As described above, according to the first aspect of the invention, asclaimed in claim 1, the carbon fiber prepreg comprises a unit layerconsisting of carbon fibers and a matrix resin layer, and steel fiberstranded filaments (including yarn strands) mixed with the carbon fibersand arranged codirectionally therewith in the unit layer. Therefore, theprepreg provides improved impact resistance and toughness by virtue ofthe steel fiber component while preserving high strength and highelasticity characteristics inherent to the carbon fiber component.Further, noticeable improvement can be achieved in the strengthcharacteristics, such as tensile and bending strength, in the axialdirection of the carbon and steel fibers. Thus, it is possible tosatisfy all of the mechanical characteristics required of a carbon fiberreinforced composite material. Such outstanding characteristics can beobtained merely through addition of such steel fiber in a slight amount,i.e., not more than 10 vol %. Therefore, it is no longer necessary toadd a large quantity of, for example, aramid fiber as has been the casein the past. Thus, it is now possible to achieve hybridization withoutdetriment to the inherent characteristics of carbon fiber, such as highelasticity, and to avoid the trouble of weight increase which hashitherto been encountered in the case of metal fibers being used. Thiscontributes to weight reduction. Further, aforesaid steel fiber exhibitsgood wettability and good adhesivity relative to the matrix resin, whichmakes it possible to avoid possible decrease in rigidity and strengthand thus to enhance quality reliability.

According to the second aspect of the invention, as claimed in claim 2,steel fiber filaments are incorporated into the matrix resin layer incodirectional relation with carbon fiber filaments. This arrangement, aswell as the claim 1 arrangement, provides good improvement in all of therequired mechanical characteristics. In this case, the steel fiber is inthe form of single filaments dispersed in a uniform and minute manner.This leads to characteristic equalization. It is also possible toincorporate steel fiber into only those portions which requirecharacteristic improvement, which fact permits efficient improvement inthe required characteristics and eventually contributes to weightreduction. Further, since steel fiber filaments are arranged in auniform and minute fashion, it is possible to positively arrest thepropagation of any crack. This can enhance overall strength improvement.In the arrangement as claimed in claim 3, sheets of above describedcarbon fiber prepreg are laminated together to form a carbon fiberreinforced resin composite. This composite can satisfy all thecharacteristic requirements of, for example, a structural component foraircrafts, when it is adopted for use in such application, and can thusmeet the foregoing demand. In the arrangement as claimed in claim 4, thesteel fiber filament is plated, and particularly where Ni plating isapplied as set forth in claim 6, such plating serves to further improvethe wettability and adhesivity of the filament surface relative to thematrix resin. Further, this lends itself to inhibition of filamentactivity in the case where a steel fiber filament having a diameter ofnot more than 100 μm, and serves to improve the self-lubricatingproperty and corrosion resistance of the filament.

In the arrangement as claimed in claim 5, the low-carbon dual-phasesteel fiber filament used as the steel fiber component has good coldworkability and can therefore be easily worked to a filament of not morethan 100 μm by suitably selecting the diameter of the wire rod to beworked and the degree of working. Further, as described earlier withrespect to the reinforcing mechanism associated with the invention, thesteel fiber filament has excellent characteristics, such as tensilestrength, elasticity, elongation and toughness. Therefore, the use ofsuch low-carbon dual-phase steel filament leads to further improvementin the required mechanical characteristics.

                                      TABLE 1                                     __________________________________________________________________________               Embodiment                                                                           Embodiment                                                             Sample 1                                                                             Sample 2                                                                             Reference Sample 1                                                                      Reference Sample 2                                                                      Reference Sample                 __________________________________________________________________________                                                 3                                Fiber content (vol %)                                                                    65.7   65.1   64.5      63.8      64.3                             Steel fiber or                                                                           1.1    4.0    1.1       1.1       0                                aramid fiber                                                                  content relative to                                                           carbon fiber (vol %)                                                          Tensile strength                                                                         1469   1540   487       1230      1201                             (MPa)                                                                         Modulus in tension                                                                       1.23   1.23   0.63      1.15      1.22                             (GPa)                                                                         Elongation at                                                                            1.09   1.10   --        0.92      0.89                             rupture (%)                                                                   Bending strength                                                                         1514   1543   822       1420      1513                             (MPa)                                                                         Modulus in 1.18   1.10   0.48      1.06      1.18                             bending (GPa)                                                                 Charpy impact                                                                            1.13   1.21   0.58      0.93      0.90                             strength (kgfm/cm.sup.2)                                                      __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________             Density Filament                                                                              Tensile Strength                                                                       Modulus of         Reduction of Area        Kind     (g/cm.sup.2)                                                                          Diameter (μm)                                                                      (kgf/mm.sup.2)                                                                         Elasticity (kgf/mm.sup.2)                                                                Elongation                                                                            at fracture              __________________________________________________________________________                                                         (%)                      Low-carbon                                                                             7.8     20      515      20000      3.9     47                       dual-phase steel                                                                       7.8     30      475      20000      4.0     53                       (superfine metal)                                                                      7.8     50      425      20000      4.2     57                       (Fe--C--Si--Mn)                                                                        7.8     100     400      20000      4.5     55                       Piano wire (82C)                                                                       7.8     100     310      20000      3.2     51                       Stainless steel                                                                        7.8     50      270      18000      3.2     60                       wire (SUS 304)                                                                __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                                     Impact Value (kgfm/cm.sup.2)                                     Test specimen  Test Value                                                                              Mean Value C.V (%)                                   ______________________________________                                        CFRP           0.95      0.95       10.2                                                     0.92                                                                          0.99                                                                          1.03                                                                          1.09                                                                          1.01                                                                          0.82                                                                          0.79                                                           Low-carbon dual-phase                                                                        1.10      1.02       --                                        steel fiber filament                                                                         1.05                                                           (1 V %)/CFRP   0.95                                                                          0.99                                                                          1.01                                                                          1.02                                                                          1.07                                                                          0.98                                                           ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________    1st-6th layer                                                                 (CF only)          7th · 9th layer (CF + low-carbon dual-phase                          steel fiber filament)                                            CF      CF   SCIFER  CF   SCIFER  SCIFER  Filament                      Sample No.                                                                          weight (g/m.sup.2)                                                                    direction                                                                          weight (g/m.sup.2)                                                                    direction                                                                          weight (g/m.sup.2)                                                                    diameter (μm)                                                                      pitch (mm)                    __________________________________________________________________________    (1)   150     ±40°                                                                     200     0°                                                                          CF only                                       (2)-1 150     ±40°                                                                     150     0°                                                                           50      50     0.3                           (2)-2 150     ±40°                                                                     150     0°                                                                           50     100     1.2                           (3)-1 150     ±40°                                                                     100     0°                                                                          100      50      0.15                         (3)-2 150     ±40°                                                                     100     0°                                                                          100     100     0.6                           __________________________________________________________________________           8th, 9th, 11th                                                                layer (CF only)                Increase                                                                              Ratio of                               CF       CF    Impact Value    ratio without                                                                         SCIFER                          Sample No.                                                                           weight (g/m.sup.2)                                                                     direction                                                                           MIN MAX Mean Value                                                                            SCIFER (%)                                                                            to CF (V %)                     __________________________________________________________________________    (1)    150      0°                                                                           1.74                                                                              1.98                                                                              1.84    --      0                               (2)-1  150      0°                                                                           1.96                                                                              2.29                                                                              2.11    15      1.4                             (2)-2  150      0°                                                                           1.95                                                                              2.26                                                                              2.09    14      1.4                             (3)-1  150      0°                                                                           2.07                                                                              2.51                                                                              2.29    24      2.8                             (3)-2  150      0°                                                                           2.03                                                                              2.41                                                                              2.25    22      2.8                             __________________________________________________________________________

What is claimed is:
 1. A carbon fiber prepreg comprising a unit layerconsisting of unidirectionally aligned carbon fibers and a matrix resinlayer, and Stranded filaments individually formed from a plurality ofsteel filaments twisted together and having a filament diameter of 100μm or less, a tensile strength of 300 to 700 kgf/mm², and an areareduction at fracture of 20% or more which are incorporated into theunit layer in codirectional relation with the carbon fibers so that thesteel fiber content of the prepreg relative to the carbon fibers is 10vol % or less.
 2. A carbon fiber prepreg comprising a unit layerconsisting of unidirectionally aligned carbon fibers and a matrix resinlayer, and single filaments of the steel fiber having a filamentdiameter of 100 μm or less, a tensile strength of 300 to 700 kgf/mm²,and an area reduction at fracture of 20% or more which are incorporatedinto the unit layer in codirectional relation with the carbon fibers sothat the steel fiber content of the prepreg relative to the carbonfibers is 10 vol % or less in such a way that they are evenly and finelydistributed within the unit layer.
 3. A carbon fiber reinforced resincomposite wherein sheets of the prepreg defined in claim 1 or 2 arelaminated together.
 4. A carbon fiber prepreg and/or carbon fiberreinforced resin composite as defined in any one of claims 1 or 2,wherein the steel fibers are plated.
 5. A carbon fiber prepreg and/orcarbon fiber reinforced resin composite as defined in any one of claims1 or 2, wherein the steel fiber is a low-carbon dual-phase steelfilament.
 6. A carbon fiber prepreg and/or carbon fiber reinforced resincomposite as defined in claim 4, wherein the plating applied to thesteel fibers is Ni plating.
 7. A carbon fiber prepreg and/or carbonfiber reinforced resin composite as defined in claim 3, wherein thesteel fibers are plated.
 8. A carbon fiber prepreg and/or carbon fiberreinforced resin composite as defined in claim 3, wherein the steelfiber is a low-carbon dual-phase steel filament.
 9. A carbon fiberprepreg and/or carbon fiber reinforced resin composite as defined inclaim 4, wherein the steel fiber is a low-carbon dual-phase steelfilament.
 10. A carbon fiber prepreg and/or carbon fiber reinforcedresin composite as defined in claim 5, wherein the plating applied tothe steel fibers is Ni plating.
 11. A carbon fiber prepreg and/or carbonfiber reinforced resin composite as defined in claim 8, wherein theplating applied to the steel fibers is Ni plating.
 12. A carbon fiberprepreg and/or carbon fiber reinforced resin composite as defined inclaim 9, wherein the plating applied to the steel fibers is Ni plating.